1. Field of the Invention
The present invention relates to a transmission/reception method for supporting Multiple Input Multiple Output (MIMO) based on feedback in a Downlink (DL) of the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). More particularly, the present invention relates to a method for restricting feedback of Precoding Matrix Indication (PMI) in 8-transmit antenna MIMO supported in LTE Release (Rel)-10.
2. Description of the Related Art
Mobile communication systems have evolved into high-speed, high-quality wireless packet data communication systems providing data services and multimedia services in addition to voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), both defined by a 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined by a 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined by the Institute for Electrical and Electronics Engineers (IEEE), have been developed to support the high-speed, high-quality wireless packet data communication services.
Recent mobile communication systems use technologies such as Adaptive Modulation and Coding (AMC) and Channel-Sensitive Scheduling to improve transmission efficiency. With the AMC method, a transmitter can adjust the amount of transmission data according to the channel state. When the channel state is bad, the transmitter reduces the amount of transmission data to adjust the reception error probability to a desired level, and when the channel state is good, the transmitter increases the amount of transmission data to adjust the reception error probability to the desired level, thereby efficiently transmitting a large volume of information. With the Channel-Sensitive Scheduling-based resource management method, the transmitter selectively services the user having a better channel state among several users, thus increasing the system capacity compared to allocating a channel to one user and servicing the user with the allocated channel. Such capacity increase is called ‘multi-user diversity gain’. The AMC method and the Channel-Sensitive Scheduling method apply an appropriate modulation and coding scheme at the most-efficient time determined depending on the partial channel state information fed back from a receiver.
Recently, intensive research is being conducted to replace Code Division Multiple Access (CDMA), the multiple access scheme used in the 2nd and 3rd generation mobile communication systems, with Orthogonal Frequency Division Multiple Access (OFDMA) in the next generation system. Standardization organizations such as 3GPP, 3GPP2, and IEEE have begun work on the evolved systems employing OFDMA. The OFDMA scheme, compared to the CDMA scheme, is expected to have an increase in capacity. One of several causes bringing about the capacity increase in the OFDMA scheme is that the OFDMA scheme can perform scheduling in the frequency domain (i.e., Frequency Domain Scheduling). As the transceiver acquires capacity gain according to the time-varying channel characteristic using the Channel-Sensitive Scheduling method, the transceiver can obtain the higher capacity gain with use of the frequency-varying channel characteristic.
In LTE, Orthogonal Frequency Division Multiplexing (OFDM) has been adopted for Downlink (DL) transmission and Single Carrier Frequency Division Multiple Access (SC-FDMA) for Uplink (UL) transmission, and both the transmission schemes are characterized by scheduling on the frequency axis.
The AMC and channel sensitive scheduling are capable of improving transmission efficiency when the transmitter has enough information on the transmit channel. In LTE DL, the base station cannot estimate a DL channel state using the UL receive channel in a Frequency Division Duplex (FDD) mode such that a User Equipment (UE) reports the information on the DL channel. However, reporting of the DL channel report of the UE to the base station can be omitted in a Time Division Duplex (TDD) mode in which the DL transmit channel state is estimated through the UL receive channel. In LTE UL, the UE transmits a Sounding Reference Signal (SRS) such that the base station estimates an UL channel using the received SRS.
In Downlink of LTE, a multiple antenna transmission technique, i.e. Multiple Input Multiple Output (MIMO), is supported. The evolved Node B (eNB) of the LTE system can be implemented with one, two, or four transmit antennas and thus can achieve beamforming gain and spatial multiplex gain by adopting precoding with the multiple transmit antennas. Since LTE Release (Rel)-10 is an advanced LTE standard, the evolved Node B (eNB) supports transmission with 8 transmit antennas.
FIG. 1 is a block diagram illustrating a configuration of an LTE eNB supporting DL MIMO according to the related art. The configuration of FIG. 1 is adopted for the operations of LTE Rel-10 system supporting transmission with 8 transmit antennas as well as the LTE system of the related art.
Referring to FIG. 1, in DL MIMO, the eNB can transmit up to two codewords 101. The codewords are transmitted in different transmission formats. The codewords are scrambled by the corresponding scramblers 103a and 103b and then modulated by the corresponding modulation mapper 105a and 105b. The modulation signals are converted to one or more signal streams 109 to be transmitted on the same frequency-time resource by the layer mapper 107. The signal streams are transmitted on the corresponding layers generated by the precoder 111. The precoded signal streams are mapped to the Resource Elements (REs) of frequency-time resource by the RE mappers 113a and 113b and then modulated to OFDM symbols by the OFDM symbol generators 115a and 115b so as to be transmitted through the transmit antenna ports 117. The controller 123 controls to determine the transmission scheme and resource such as the modulation scheme, number of layers, precoding scheme, and RE allocation, based on the feedback information received by means of the feedback receiver 119. The feedback information includes the DL channel state reported by the UE.
FIG. 2 is a block diagram illustrating a configuration of an LTE UE supporting DL MIMO according to the related art. The configuration of FIG. 2 is adopted for the operations of LTE Rel-10 system supporting transmission with 8 transmit antennas as well as the LTE system of the related art.
Referring to FIG. 2, the UE converts the Radio Frequency (RF) signal received through the receive antennas 201 in baseband signals by means of the RF receivers 203a and 203b. The Reference Signal (RS) carrying the DL channel information is extracted from the converted baseband signal. The channel estimator 205 uses the RS to estimate the DL channel. Physical Downlink Control Channel (PDCCH) and Physical Downlink Shared Channel (PDSCH) are recovered by the PDCCH/PDSCH receiver 207. The signaling information transmitted by the eNB through PDCCH and PDSCH is delivered to the controller 211 such that the controller 211 saves the eNB instruction in the memory 213. The channel estimation value obtained by the channel estimator 205 is used for demodulating PDSCH/PDCCH and generating feedback information by the feedback information generator 209. The feedback information generator 209 generates the feedback information, such as Channel Quality Indication (CQI), Precoding Matrix Indication (PMI), and Rank Indication (RI), and transmits the feedback information on Physical Uplink Control Channel (PUSCH). Since the Single Carrier Frequency Domain Multiple Access (SC-FDMA) is adopted in LTE UL, the feedback information is carried in the PUSCH. Table 1 shows the DL transmission modes defined in LTE Rel-8 and Rel-9.
TABLE 1DL Transmission (TX) modes supported in LTE Rel-8 and Rel-9TransmissionmodeDescription1Single-antenna port, port 02Transmit diversity3Open-loop spatial multiplexing4Closed-loop spatial multiplexing5Multi-user MIMO6Closed-loop spatial multiplexing using asingle transmission layer7Single-antenna port; port 58Dual layer transmission; port 7 or 8 or both
In the LTE system, the transmission port is defined by the RS used in modulation. In LTE DL, the RS associated with the transmission port p is transmitted through the antenna port p. The set of transmit/antenna port p is composed differently according to the RS configuration of the corresponding eNB.
Cell-specific RS (CRS) is defined for the eNB using 1, 2, or 4 transmit antennas corresponding to the antenna ports of p=0, p={0,1}, and p={0, 1, 2, 3}.
Multicast Broadcast Single Frequency Network (MBSFN) RS corresponds to the antenna port of p=4.
DeModulation RS (DM-RS) as UE-specific RS corresponds to the antenna port of p=5 in transmission mode 7 and p=7, p=8, or p={7, 8} in transmission mode 8.
The transmission modes 1 to 6 support the CRS-based transmission schemes. For example, the transmission modes 3 and 4 support spatial multiplexing with the CRS as the reference signal for demodulation. The transmission modes 7 and 8 use the DM-RS for demodulation. In order to support the closed-loop MIMO, the UE estimates the DL MIMO channel with CRS and reports CQI, PMI, and RI to the eNB. The CQI is referenced by the eNB to determine Modulation and Coding Scheme (MCS), and the PMI and RI are referenced by the eNB to determine the precoding and number of MIMO transmission layers. The eNB makes a final determination on the transmission scheme, precoding scheme, and transmission resource for transmission of a PDSCH as a DL data channel based on the feedback information such as CQI, PMI, and RI.
In order to perform the closed-loop operation normally, the UE and eNB should interpret the feedback information identically. The LTE system uses a standardized codebook for precoding to define the feedback information of PMI and RI. Table 2 shows the codebook used in the LTE system with two transmit antennas.
TABLE 2precoding codebook for LTE system with two TX antennasCodebookNumber of layersindex120      1          2        ⁡      [                            1                                      1                      ]        1          2        ⁡      [                            1                          0                                      0                          1                      ]   1      1          2        ⁡      [                            1                                                  -            1                                ]        1    2    ⁡      [                            1                          1                                      1                                      -            1                                ]   2      1          2        ⁡      [                            1                                      j                      ]        1    2    ⁡      [                            1                          1                                      j                                      -            j                                ]   3      1          2        ⁡      [                            1                                                  -            j                                ]  —
A precoding matrix is selected from Table 2. However, the matrix
  W  =            1              2              ⁡          [                                    1                                0                                                0                                1                              ]      is the precoding matrix only for the open-loop spatial multiplexing.
Table 3 shows the codebook used in the LTE system with four transmit antennas. Wn{s} is the matrix defined by the column vectors given by the set {s} as shown in the following equation:
      W    n    =      I    -          2      ⁢                                    u            n                    ⁢                      u            n            H                                                u            n            H                    ⁢                      u            n                              where I denotes a 4×4 unitary matrix, and un denotes the value given in Table 3.
TABLE 3precoding codebook for LTE system with four TX antennasCode-bookNumber of layers υindexun12340uo = [1 −1 −1 −1]Tw0{1}w0{14}/{square root over (2)}w0{124}/{square root over (2)}w0{1234}/{square root over (2)}1u1 = [1 −j 1 j]Tw1{1}w1{12}/{square root over (2)}w1{123}/{square root over (2)}w1{1234}/{square root over (2)}2u2 = [1 1 −1 1]Tw2{1}w2{12}/{square root over (2)}w2{123}/{square root over (2)}w2{3214}/{square root over (2)}3u3 = [1 j 1 −j]Tw3{1}w3{12}/{square root over (2)}w3{123}/{square root over (2)}w3{3214}/{square root over (2)}4u4 = [1 (−1 − j)/w4{1}w4{14}/{square root over (2)}w4{124}/{square root over (2)}w4{1234}/{square root over (2)}{square root over (2)} −j (1 − j)/{square root over (2)} ]T5u5 = [1 (−1 − j)/w5{1}w5{14}/{square root over (2)}w5{124}/{square root over (2)}w5{1234}/{square root over (2)}{square root over (2)} j (−1 − j)/{square root over (2)} ]T6u6 = [1 (1 ÷ j)/{square root over (2)} − j w6{1}w6{13}/{square root over (2)}w6{134}/{square root over (2)}w6{1324}/{square root over (2)}(−1 + j)/{square root over (2)} ]T7u7 = [1 (−1 ÷ j)/w7{1}w7{13}/{square root over (2)}w7{134}/{square root over (2)}w7{1324}/{square root over (2)}{square root over (2)} j (1 ÷ j)/{square root over (2)} ]T8u8 = [1 −1 1 1]Tw8{1}w8{12}/{square root over (2)}w8{124}/{square root over (2)}w8{1234}/{square root over (2)}9u9 = [1 −j −1 −j]Tw9{1}w9{14}/{square root over (2)}w9{134}/{square root over (2)}w9{1234}/{square root over (2)}10u10 = [1 1 1 −1]Tw10{1}w10{13}/{square root over (2)}w10{123}/{square root over (2)}w10{1324}/{square root over (2)}11u11 = [1 j −1 j]Tw11{1}w11{13}/{square root over (2)}w11{134}/{square root over (2)}w11{1324}/{square root over (2)}12u12 = [1 −1 −1 1]Tw12{1}w12{12}/{square root over (2)}w12{123}/{square root over (2)}w12{1234}/{square root over (2)}13u13 = [1 −1 1 −1]Tw13{1}w13{13}/{square root over (2)}w13{123}/{square root over (2)}w13{1324}/{square root over (2)}14u14 = [1 1 −1 −1]Tw14{1}w14{13}/{square root over (2)}w14{123}/{square root over (2)}w14{3214}/{square root over (2)}15u15 = [1 1 1 1]Tw15{1}w15{12}/{square root over (2)}w15{123}/{square root over (2)}w15{1234}/{square root over (2)}
Precoding used in DL MIMO channel is a significant technology for obtaining beamforming gain and spatial multiplexing gain. The UE reports the channel state of the given DL MIMO channel to the eNB with the most appropriate PMI and RI. PMI is the value indicating the precoding matrix requested by the UE, and RI is the value indicating the maximum number of layers for transmitting signals simultaneously in the current channel state that is determined by the UE.
However, the eNB cannot accept a precoding matrix and rank selected by the UE. For example, the eNB should avoid a precoding matrix and rank that causes significant interference to the neighbor cells. The eNB also may not support some precoding matrices due to the high transmitter complexity or may restrict the PMI and RI selectable by the UE due to the lack of the reliability on the feedback information from the UE.
In order for the eNB to restrict PMI and RI fed back from the UE, a Codebook Subset Restriction (CSR) technique is introduced in LTE Rel-8 and Rel-9. A CRS bitmap is sent to each UE by upper layer signaling. A specific bit of the bitmap matches the corresponding precoding matrix. A specific bit is set to 0 in the CRS bitmap, and the precoding matrix corresponding to the bit is restricted such that the UE feedback on the restricted matrix is blocked. The size of the CRS bitmap is identical with the total number of precoding matrices defined in the standard and determined depending on the transmission mode of the UE and number of CRS antenna ports of the eNB.
FIG. 3 is a signaling diagram illustrating a closed-loop precoding procedure with CSR according to the related art.
Referring to FIG. 3, the eNB 301 first performs codebook subset restriction signaling to the UE 303 in step 305. The UE stores the CSR bitmap and, when feedback is necessary, determines CQI, PMI, and RI by referencing the CSR bitmap in step 309. The feedback information generated in step 309 is transmitted to the eNB in step 311. The eNB perform DL scheduling in step 313 based on the feedback information, and transmits PDCCH and PDSCH in step 315. The UE receives PDCCH to acquire the information on PDSCH in step 317 and receives the PDSCH in step 319. The process 307 including steps 309 to 319 shows the operations of the eNB and UE for normal closed-loop DL transmission. The CSR signaling update 305 is not necessary for every feedback and data reception process 307. The CSR signaling can be performed by the eNB when CSR update is necessary.
In LTE Rel-8 and Rel-9, the CSR is supported in transmission modes 3, 4, 5, 6, and 8. The CSR bitmap sizes in the individual transmission modes are summarized in Table 4.
TABLE 4CSR bitmap size per transmission modeNumber of bits ACTransmission2 antenna4 antennamodeportsports3244664541664168632
The CSR bitmap is expressed with a bit stream of {aAC−1, . . . , a3, a2, a2, a1, a0}. Here, a0 is the Least Significant Bit (LSB), and aAC−1 is the Most Significant Bit (MSB).
Transmission Mode 4 is the transmission mode for the closed-loop MIMO based on the DL CRS defined in LTE Rel-8. In transmission mode 4, the total number of precoding matrices defined in 2-TX codebook is 6 such that the bitmap size of 6 is used for transmission mode 4 CSR signaling of the LTE system supporting two transmit antennas. The total number of precoding matrices defined in 4-TX codebook is 64 such that the bitmap size of 64 is used for transmission mode 4 CSR signaling of the LTE system supporting four transmit antennas.
Transmission Mode 8 is the transmission mode for dual beamforming based on the DL DM-RS added in LTE Rel-9. In transmission mode 8, only rank-1 or rank-2 transmission is supported. In transmission mode 8, the total number of precoding matrices defined in 4-TX codebook is 32 such that the bitmap size of 32 is used for the transmission mode 8 CRS signaling of the LTE system supporting four transmit antennas.
Each bit of the CRS signaling bitmap per transmission mode is interpreted as follows:
Transmission Mode 3
2 TX antennas: Bit av−1, v=2 is designated for the precoding matrix corresponding to the codebook index i of Table 2 and rank 2. Here, a0 denotes the precoding for transmission diversity.
4 TX antennas: Bit av−1, v=2, 3, 4 is designated for the precoding matrix corresponding to the codebook indices 12, 13, 14, and 15 in Table 3 and rank v. Here, a0 denotes the precoding for transmission diversity.
Transmission Mode 4
2 TX antennas: Refer to Table 5
4 TX antennas: Bit a16(v−1)+i is designated for the precoding matrix corresponding to the codebook index i in Table 3 and rank v.
Transmission Modes 5 and 6
2 TX antennas: Bit ai is designated for the precoding matrix corresponding to the codebook index i in Table 2 and rank 1.
4 TX antennas: Bit ai is designated for the precoding matrix corresponding to the codebook index i in Table 3 and rank 1.
Transmission Mode 8
2 TX antennas: Refer to Table 5
4 TX antennas: Bit a16(v−1)+i is designated for the precoding matrix corresponding to the codebook index in Table 3 and rank v, where v=1,2.
Table 5 shows the summarized rule for interpreting the CSR bit map in transmission modes 4 and 8 with two transmit antennas.
TABLE 5relationship between CSR bitmap andprecoding matrix in 2-TX codebookCode bookNumber of layersindex120a0—1a1a42a2a53a3—
In the technique of the related art, the CSR bitmap is defined to match the bits to the precoding matrices available in a specific transmission mode one by one. In LTE Rel-8 and Rel-9, the conventional method can support the use of CRS since only one precoding codebook is defined per transmission mode.
In LTE Rel-10 which first supports 8 transmit antennas, however, the 8-TX codebook is newly defined. With the increase of the number of transmit antennas, the width of the beam formed by precoding becomes narrow, resulting in increase of array antenna gain. Improved array antenna gain can actually be obtained only when the eNB can receive the more accurate PMI feedback information. In LTE Rel-10, a dual codebook structure is adopted for new 8-TX codebook structure for defining the PMI feedback information without large increase of feedback overhead.
With the introduction of the newly structured precoding codebook, the conventional CSR bitmap signaling method cannot be reused any more. There is therefore a need of the efficient CSR signaling method to address this issue.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.